Multiple stage, multiple extend, speed reducing ball screw linear actuator and method of constructing and operating the actuator

ABSTRACT

A multiple extend ball screw and nut linear actuator incorporates a revolvable axially restrained ball screw with a ball nut thereon. A constant speed reversible drive revolves the screw to move the ball nut forwardly and rearwardly. External helical threads of the same hand as the ball nut threads are provided externally on the ball nut but have a substantially reduced lead. A second rotatably restrained nut having threads matching the external threads on the ball nut mounts on the ball nut and connects to the system to be actuated.

[0001] This application claims the priority of U.S. provisionalapplication 60/200,120, filed Apr. 27, 2000.

[0002] This invention relates to ball screw linear actuators and, moreparticularly, to multiple stage, sequenced, multiple extend actuatorswith end of stroke decelerated speed.

BACKGROUND OF THE INVENTION

[0003] Conventionally, variable speed motor input drives, are utilizedto drive a primary stage ball screw and move a ball nut the greaterproportion of the actuator's linear stroke at a relatively high speed,and then are cushioned to abruptly decelerate during the end portion ofthe stroke. This is normal practice for multiple stage actuators, suchas the type shown in the present assignee's U.S. Pat. 3,762,227, inwhich there is a multiple extension of the actuator components instages, but no provision for providing automatic “end of stroke”speedreduction.

SUMMARY OF THE INVENTION

[0004] It is desirable that the actuator be improved so that the drivemotor utilized can provide a constant speed input. The inventionprovides an elongate, revolvable, axially restrained ball screw andaccompanying ball nut having thread forming helical land and grooveportions which provide a raceway between them for load bearing balls,wherein the ball nut travels a major portion of the stroke of themultiple extend linear actuator. The ball nut is capable of rotation, aswell as translation along the ball screw.

[0005] An actuator nut, with internal threads, is mounted on the ballnut, which is provided with matching external threads. The hand of thethreads of the actuator nut is the same as the hand of the threads ofthe ball nut, but the actual nut threads have a shorter lead by at leasta factor of two. The ball screw is provided with a resilient energydissipating stop which extends into the path of the ball nut to firstdecelerate the travel of the ball nut and then to effect a coupling ofthe screw shaft and ball nut so that they begin rotating as anintegrated assembly at the constant speed of the motor. This causes theactuator nut to extend, but at a significantly reduced speed. Thereversing stroke of the actuator is similar in that the major portion ofthe reversing travel is created by the ball nut traveling along the ballscrew, the ball screw then being stopped and rigidly coupled with theball screw to initiate travel of the actuator nut in the reversedirection.

[0006] It is a prime object of the present invention to be able to todrive the ball screw at a constant speed during both the forwarding andreversing travel of the linear actuator.

[0007] It is a further object of the invention to provide an actuatorassembly which is so constructed as to provide a linear deceleration ofthe ball nut and to effect a coupling of the ball nut to the screw shaftso that the ball screw and the ball nut rotate as a rigid couple whichmoves the actuate member being actuated at a much reduced speed at theend of the actuator stroke.

[0008] Another object of the invention is to provide a relativelydurable, maintenance free, multiple stage actuator system which operatessmoothly and reliably to effect an actuation of the system to which theactuator nut is connected.

[0009] Another object of the invention is to provide an economicallyavailable actuator and drive which does not impose undue loads on thesystem which it operates.

[0010] Other objects and advantages of the invention will becomeapparent with reference to the accompanying drawings and theaccompanying descriptive matter.

BRIEF DESCRIPTON OF THE DRAWINGS

[0011] The presently preferred embodiment of the invention is disclosedin the following description and in the accompanying drawings, wherein:

[0012]FIG. 1 is a schematic, sectional, side elevational viewillustrating the various actuator components in the start position;

[0013] FIGS. 2 is a similar view illustrating the ball nut in a forwardposition in which it is sequencing to revolving travel with the ballscrew;

[0014]FIG. 3 is a similar view showing the actuator nut as having movedforwardly to a final actuating position;

[0015]FIG. 4 is a view similar to FIGS. 1-3, but with the ball nutreversed to original position and the actuator nut ready to commence itstranslatory travel in a reverse direction on the ball nut; and

[0016]FIG. 5 depicts a typical graph depicting the actuator stroke.

GENERAL DESCRIPTION OF THE INVENTION

[0017] Referring now more particularly to the accompanying drawings, theactuator assembly in the present invention comprises a typical ballscrew 11 provided with helical ball accommodating groove portions 12separated by land or thread portions 13. Axially provided on the ballscrew 11 is a ball nut, generally designated 14, which has matchinginternal helical groove portions 15 of the same hand and the same pitchor lead as the groove portions 12, and which are separated by helicalland portions 16. Provided in the nut to recirculate the abutting loadbearing balls 17, which travel in the helical raceway R, is the usualball return passage 18.

[0018] It will be noted that the external surface of the ball nut 14 isexternally threaded as at 19 with threads of the same hand as the ballnuts internal threads, but a greatly reduced pitch or lead, and that anactuator nut 20, having matching internal threads 20 a, is mounted fortranslatory travel thereon. It is the actuator nut 20 which is connectedto the member 21 to be actuated, which, in the present case for purposesof convenience only, is shown as a lever member 21 pivotally connectedto the nut 20 as at 22.

[0019] As shown the ball screw 11 has a reduced shaft portion 23 onwhich a suitable drive gear 24 is provided. Typically the gear 24, whichdrives the revolveable screw 13 in rotation, may be driven through agear box or the like by a reversible constant speed electric motor M. Ateach end of the ball screw 11, a reduced shaft portion 25 of the ballscrew 12 is provided with a threaded end 26 to accommodate a stop nut27. While the nut 27 at each end of the ball screw 11 rotates with theball screw 11, it may be so mounted to a frame F for the system so as toprevent the ball screw 11 from moving axially.

[0020] Provided on the reduced portions 25 at each end of the ball screw11 are resilient spring damping assemblies, generally designated 28,which can comprise inner end washers 29 and energy absorbing ringsprings 30. It will be observed that the threads 14 and 19 arepreferable matching Acme threads, which have a significantly greaterfrictional resistance to travel of the actuator nut 20 on the ball nut14 than does the ball screw 14 on the elongate ball screw 11. It willfurther be noted that the actuator nut 20 can mount optionalfriction-increasing elements, such as a set screw 31, to increase thecoefficient of frictional resistance as desired.

[0021] The nuts 27, it will be observed, can mount resilient springassemblies 32 in the path of the actuator nut 20. These are shown ascoil springs received within spring mount groove 33 but may be of anysuitable configuration.

[0022]FIG. 5 is a graphical representation of a typical actuator strokein which linear velocity is plotted against travel. It will be seen thatthe travel “x”in FIG. 5 of the ball nut 14 is depicted as having thespeed value “y”and the travel of the actuator nut 20 is depicted ashaving the speed value “y/4”.

THE OPERATION

[0023] The majority of travel of the actuator lever 21 occurs with theball nut 14 moving translatorily on the ball screw 11 forwardly tocontact the spring assembly 28. While the spring assembly 28 is shown asa ring spring assembly, it could consist of resilient rubber pads or asystem of coil springs.

[0024] When the ball nut 14 impinges upon the washer 29, the translatoryforward travel of the ball nut 14 is sharply reduced as shown in FIG. 5by the deceleration line “z”. When the ball nut 14 almost reaches avirtual stop, it automatically couples to the ball screw 11 andcommences to revolve. The rotatably restrained actuator nut 20 thencompletes the travel of lever 21 at a much reduced “y/4”speed. While thespeed “y”in FIG. 5 has been decreased by a factor of 4 near the end ofthe actuator stroke, it is to be understood that other factors ofdecrease may be employed. The speed will be reduced, however, by atleast a factor of 2. When the lever 21 is to be returned to originalposition, the motor M is reversed and the ball screw 11 is driven in areverse direction of rotation. The return trip is also depicted by thegraph in FIG. 5 in that the major portion of the trip at higher speedwill be accomplished by the ball nut 14 returning to the position inwhich it is shown in FIG. 4, where it is stopped in the same manner bythe stop assembly 28 at the left end of the ball screw in FIGS. 1-4.Thereafter, the ball screw and ball nut 14 will be automatically coupledfor rotation and the actuator nut 20, which may be prevented fromrotating by the lever 21 or in another manner, will be moved from rightto left to the position shown in FIG. 1.

[0025] While, conceivably, the actuator nut 20 could be a ball nut withhelical ball grooves and lands, matching with like groove and landsprovided on the exterior of the ball nut 14, the Acme threads at 19 and20 a provide far more frictional resistance which acts to facilitate thereduction of speed at the end of the stroke. The Acme threads chosen,and the addition of one or more efficiency altering adjustable elements31, can be varied to provide best results for the particular operation.The invention provides a compound automatic linear deceleration at endof stroke both during extension and retraction, eliminating the need forany motor speed control.

[0026] The disclosed embodiment is representative of a presentlypreferred form of the invention, but is intended to be illustrativerather than definitive thereof. The invention is defined in the claims.

We claim:
 1. A multiple stage, multiple extend, end of stroke speedreducing, ball screw and nut linear actuator comprising, in combination:a. a ball screw having helical groove and land portions with a first nutmounted thereon having complemental helical groove portions defining aball raceway system between the screw and first nut connected withrecirculant ball passages; b. said raceway system having a predeterminedhand and lead, and incorporating load bearing balls; c. a mount systemfor said screw restraining its axial movement while permitting itsrotation; d. a constant speed reversible drive connected to revolve saidscrew in one direction at a constant speed and in the reverse direction,and move said first nut forwardly and rearwardly respectively; e.external helical groove and land portions of the same hand as said nutgroove portion on said first nut but of a substantially reduced lead; f.a second rotatably restrained nut having helical internal groove andland portions complementing said external groove and land portions onsaid first nut; and g. yieldable members on said screw in the path ofsaid first nut for halting travel of said first nut axially and causingcoupling of said ball screw and first nut and rotation of said first nutat said constant speed to effect axial movement of said second nut inthe same direction of travel at a significantly reduced speed.
 2. Theactuator of claim 1 wherein said stop members are provided at each endof said screw and comprise resilient energy absorbing members providinga speed deceleration zone of travel for said actuator.
 3. The actuatorof claim 2 wherein said stops comprise spring washers providing aresiliently resisted zone of travel for said first nut.
 4. The actuatorof claim 1 wherein said external groove portions on said first nut andsaid internal land groove portions on said second nut are acme threads.5. The actuator of claim 1 wherein said land and groove portions on saidfirst nut and second nut are of a pitch on the order of one quarter ofthe pitch of the land and groove portions of said ball screw.
 6. Theactuator of claim 1 wherein said groove and land portions on said secondand first nuts mount adjustable friction increasing members.
 7. Theactuator of claim 1 wherein said friction increasing members compriseset screws.
 8. The actuator of claim 1 wherein resilient stop membersare provided opposite each end of said second nut in the path thereof.9. The actuator of claim 1 wherein said reduced lead of the land andgroove portions of said first and second nuts is at least half the leadof said land and groove portions of said ball screw.
 10. A two stagelinear actuator comprising: a. a first stage threaded elongate ballscrew member with a ball nut member mounted thereon, there being firstthreads of a predetermined hand and lead provided to couple said membersfor relative translatory movement; b. mechanism resiliently halting saidtravel of said members to provide a speed reduction zone of travel andthen coupling said members for conjoint rotary travel; and c. a secondrotatably restrained threaded nut member mounted on said ball nut memberfor axial travel thereon, there being second threads between said nutmembers of the same hand as said first threads, but a significantlyreduced lead relative to said first threads.
 11. A method ofconstructing a two stage actuator incorporating a first ball screw witha ball nut thereon, said ball screw and ball nut having threads of afirst hand and lead providing a ball raceway; there being a secondrotatably restrained nut threaded on said ball nut; comprising: a.loading balls to said raceway; b. providing mechanism revolvablymounting but axially restraining said ball screw; c. providing resilientstops for said ball nut for first decelerating travel of said ball nutand them coupling said ball screw and ball nut for conjoint rotation;and d. attaching a member to be actuated to said second nut for linearaxial travel therewith.
 12. The method of claim 11 comprising providingfriction adjusting elements on said second nut to frictionally resistits travel on said ball nut and adjusting the position of said elements.13. A method of operating a two stage actuator comprising a ball screwwith a ball nut threaded thereon to provide a helical raceway betweenthem having load bearing balls therein, the threading having a hand andlead; and there being an actuator nut attached to the system to beoperated threaded on said ball nut for axial travel thereon, thethreading of said actuator nut being of the same hand but of asignificantly reduced lead relative to said ball nut threading;comprising: a. revolving said ball screw at a constant speed in onedirection of rotation while restraining its axial travel to move saidball nut axially in a forwarding direction; b. resiliently blockingaxial travel of said ball nut after a predetermined movement thereof anddecelerating its travel before coupling it to said screw for conjointrotation therewith; c. utilizing the rotation of said ball nut to drivesaid actuator nut axially forwardly in the same direction of axialtravel at a significantly reduced speed.
 14. The method of claim 13wherein said actuator nut is moved at a speed reduced at least in half.15. The method of claim 13 wherein steps a-c are followed in a reversingdirection when said ball screw is rotated in the reverse direction.